For various electronic devices, a clean and continual power supply is a sine qua non for maintaining a normal performance. To prevent the faulty operation of an electronic device as a result of the interruption or abnormality of the AC power provided by an ordinary power source, an uninterruptible power supply is developed to provide backup power to maintain the normal operation of an electronic device while the input AC power of the electronic device can not be supplied normally.
FIG. 1 illustrates a circuit diagram showing the construction of a conventional on-line uninterruptible power supply, in which an input AC voltage Vin is supplied by a utility power source or a commercial power source. As shown in FIG. 1, a conventional on-line UPS is made up of a circuit breaker 110, a boost rectifier circuit 120, a battery 130, and an inverter circuit 140, in which the boost rectifier circuit 120, the battery 130, and the inverter circuit 140 are connected together through buses 10a and 10b. The circuit breaker 110 is coupled between a voltage input terminal 101 and the boost rectifier circuit 120 for receiving the input AC voltage Vin. The boost rectifier circuit 120 is coupled between the circuit breaker 110 and the input terminals of the inverter circuit 140 for rectifying the input AC voltage Vin and generating boosted DC voltages on buses 10a and 10b. The boost rectifier circuit 120 includes a rectifier 121 formed by bridge-connected rectifying diodes for performing a half-wave rectification or full-wave rectification to the input AC voltage Vin so as to generate a rectified DC voltage with a predetermined voltage level. The boost rectifier circuit 120 further includes: a positive boost converter 122 including a current monitor CTA1, a boost inductor L11, a transistor switch Q11, and a diode D11; a negative boost converter 123 including a current monitor CTA2, a boost inductor L12, a transistor switch Q12, and a diode D12; output capacitors C11 and C12; and a current-limiting circuit 124. The positive boost converter 122 is coupled to a positive output terminal of the rectifier 121, in which the boost inductor L11 is configured to receive a current from the rectifier 121 during the positive half-cycle of the input AC voltage and discharge a current to the output capacitor C11 through the diode D11 according to the on/off operations of the transistor switch Q11, and thereby generating a positive DC voltage across the output capacitor C11. The negative boost converter 123 is coupled to a negative output terminal of the rectifier 121, in which the boost inductor L12 is configured to receive a current from the rectifier 121 during the negative half-cycle of the input AC voltage and discharge a current to the output capacitor C12 through the diode D12 according to the on/off operations of the transistor switch Q12, and thereby generating a negative DC voltage across the output capacitor C12. The current-limiting circuit 124 is made of a first silicon-controlled rectifier (or SCR) 151 and a second silicon-controlled rectifier 152 that are connected in parallel with each other and manipulated by a microcontroller (not shown) to allow the battery 130 provide a current to the boost inductors L11 and L12 when the input AC voltage is abnormal or interrupted. Therefore, the battery 130 can respectively provide a positive DC voltage and a negative DC voltage to the input terminal of the positive boost converter 122 and the input terminal of the negative boost converter 123, and thereby respectively generating a regulated output voltage across the output capacitor C11, C12 under the manipulation of the microcontroller. In addition, a resistor R11 is connected in series between the bus 10a and the first silicon-controlled rectifier 151 for protecting the battery 130 from the negative electrochemical effect as a result of the surging inrush current and thereby preventing the lifetime of the battery 130 from being shortened.
The purpose of the battery 130 is to provide backup power to continue supplying power to a load (not shown) when the input AC voltage Vin is abnormal or interrupted. When the input AC voltage Vin is normal, a battery charger (not shown) can be used to charge the battery 130 through the bus voltage provided by the buses 10a and 10b. When the input AC voltage Vin is abnormal or interrupted, the battery 130 can provide backup power. The current monitors CTA1 and CTA2 which are implemented by Hall effect current sensors are respectively mounted on the bus 10a, 10b and respectively coupled to the boost inductor L11, L12. The current monitors CTA1 and CTA2 are configured to measure the output current on the buses 10a and 10b, and the measured output current is provide for the microcontroller as a reference to activate the current sharing function and the over-current protection. The inverter circuit 140 is coupled to the output terminals of the boost rectifier 120 and includes a ladder of switches (Qa1, Qb1, Qc1, Qd1), a first output filter (La1, Ca1), and a second output filter (Lb1, Cb1). The inverter circuit 140 is configured to convert the positive output voltage and the negative output voltage of the boost rectifier circuit 120 into an AC voltage according to the on/off operations of the switches (Qa1, Qb1, Qc1, Qd1), and thereby generating a poly-phase AC voltage through the regulation of the first output filter (La1, Ca1) and the second output filter (Lb1, Cb1). The resultant poly-phase AC voltage is provided to a load (not shown) through output terminals 160.
FIG. 2 is a timing diagram showing the control signal waveforms used in the uninterruptible power supply of FIG. 1. Next, the operation of the uninterruptible power supply of FIG. 1 will be explored with reference to the timing diagram of FIG. 2. When the input AC voltage Vin is normal, that is, when the uninterruptible power supply 100 is working in the AC mode, the input AC voltage Vin is rectified so as to provide a positive DC voltage to the input terminal of the positive boost converter 122 and provide a negative DC voltage to the input terminal of the negative boost converter 123. By driving the transistor switches Q11 and Q12 with a 40-Hz switching control signal issued from the microcontroller, the positive boost converter 122 may generate a positive DC voltage across the output capacitor C11 and the negative boost converter 123 may generate a negative DC voltage across the output capacitor C12. When the input AC voltage Vin is abnormal or interrupted, that is, when the uninterruptible power supply 100 is working in the backup mode, the first silicon-controlled rectifier 151 is turned on under the manipulation of the microcontroller such that the battery 130 may respectively output a positive DC voltage and a negative DC voltage to the input terminal of the positive boost converter 122 and the input terminal of the negative boost converter 123. The positive DC voltage and the negative DC voltage outputted from the battery 130 are respectively boosted by the boost inductor L11 and the boost inductor L12, and thereby respectively generating an output DC voltage across the output capacitor C11 and the output capacitor C12 according to the on/off operations of the transistor switches Q11 and Q12. Because the first silicon-controlled rectifier 151 is connected in series with the resistor R11, the resistor R11 may suppress the surge of the inrush current to achieve a smooth soft-start for the boost rectifier circuit 120. When the voltage on the buses 10a and 10b reaches a predetermined level, the second silicon-controlled rectifier 152 is turned on and the inverter circuit 140 converts the output voltages of the boost rectifier circuit 120 into a regulated AC voltage for use by a load.
However, the foregoing uninterruptible power supply 100 has some drawbacks needing to be solved. For example, when the uninterruptible power supply 100 is working in the backup mode, the DC currents outputted from the battery 130 may flow through the boost inductors L11 and L12. Because the boost inductors L11 and L12 are magnetic elements, a considerable power loss would generate within the uninterruptible power supply 100. Therefore, the conversion efficiency of the boost rectifier circuit 120 would be deteriorated. Furthermore, each bus requires to a current monitor being implemented by semiconductor devices to be placed thereon to measure the output current of the buses 10a and 10b. This would increase the manufacturing cost of the uninterruptible power supply.
It is therefore a tendency to develop an uninterruptible power supply with low cost and low power loss to address the above-mentioned problems.